1. If you cross a pony with a human, you get a satyr. Go ahead and cross them with other things if you like! 2. 'Main Tribes' has no mechanical meaning. Earth-Bound, Unicorn, and pegasus are all physical tribes, just like the others your listed. 2a. Be an earth-bound, that's a valid choice. See abov...

Neither works the way you think it does. They're just two halves of what makes a thing, a thing. I tried to err towards physical outward things being 'bodied' and more subtle magicy things being 'blooded', but it has no mechanical meaning at all. It could be 'Aboleth's Cookie' and mean the same thing.